This application claims priority from Finnish Patent Application No. 19992268, entitled xe2x80x9cInternal Antenna for an Apparatus,xe2x80x9d filed on Oct. 20, 1999, the disclosure of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The invention relates to an antenna structure to be installed inside sm radio apparatus.
2. Description of Related Art
In portable radio apparatus it is very desirable that the antenna be located inside the covers of the apparatus, for a protruding antenna is impractical. In modem mobile stations, for example, the internal antenna naturally has to be small in size. This requirement is further emphasized as mobile stations become smaller and smaller. Furthermore, in dual-band antennas the upper operating band at least should be relatively wide, especially if the apparatus in question is meant to function in more than one system utilizing the 1.7-2 GHz band.
When aiming at a small-sized antenna the most common solution is to use a PIFA (planar inverted F antenna). The performance, such as bandwidth and efficiency, of such an antenna functioning in a given frequency band or bands depends on its size: The bigger the size, the better the characteristics, and vice versa. For example, decreasing the height of a PIFA, i.e. bringing the radiating plane and ground plane closer to each other, markedly decreases the bandwidth. Likewise, reducing the antenna in the directions of breadth and length by making the physical lengths of the elements smaller than their electrical lengths especially degrades the efficiency.
FIG. 1 shows an example of a prior-art dual-band PIFA. Depicted in the figure is the frame 110 of the apparatus in question which is drawn horizontal and which functions as the ground plane of the antenna. Above the ground plane there is a planar radiating element 120 supported by insulating pieces, such as 105. Between the radiating element and ground plane there is a short-circuit piece 102. The radiating element 120 is fed at a point F through a hole 103 in the ground plane. In the radiating element there is a slot 125 which starts from the edge of the element and extends to near the feed point F after having made two rectangular turns. The slot divides the radiating element, viewed from the feed point F, into two branches A1 and A2 which have different lengths. The longer branch A1 comprises in this example the main part of the edge regions of the radiating element, and its resonance frequency falls on the lower operating band of the antenna. The shorter branch A2 comprises the middle region of the radiating element, and its resonance frequency falls on the upper operating band of the antenna. The disadvantage of structures like the one described in FIG. 1 is that the tendency towards smaller antennas for compact mobile stations will in accordance with the foregoing degrade the electrical characteristics of an antenna too much.
The object of the invention is to reduce the aforementioned disadvantages associated with the prior art. The structure according to the invention is characterized by what is expressed in the independent claim 1. Preferred embodiments of the invention are presented in the other claims.
The basic idea of the invention is as follows: A conventional PIFA type structure is extended is such a manner that instead of one there will be at least two radiating planes on top of each other above the ground plane. Between them there is dielectric material in order to reduce the size of the lower radiator and to improve band characteristics. Likewise, there is dielectric material on top of the uppermost radiating plane. This top layer is used to bring one resonance frequency of the antenna relatively close to another resonance frequency in order to widen the band. The upper radiating plane is advantageously galvanically connected to the lower radiating plane.
An advantage of the invention is that it achieves a greater increase in the antenna bandwidth than what would be achieved by placing the only radiating plane at a distance from the ground plane equal to that of the upper radiating plane according to the invention. This is due to the use of multiple resonance frequencies close to each other. Other advantages of the invention include relatively good manufacturability and low manufacturing costs.
The invention will now be described in detail. Reference will be made to the accompanying drawings in which